Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,456	$ 1,334
Due after one year through five years, cost or amortized cost	12,304	9,746
Due after five years through ten years, cost or amortized cost	12,319	14,892
Due after ten years, cost or amortized cost	12,847	12,134
Cost or Amortized Cost	38,926	38,106
Due in one year or less, estimated fair value	1,458	1,356
Due after one year through five years, estimated fair value	13,098	10,186
Due after five years through ten years, estimated fair value	13,878	15,931
Due after ten years, estimated fair value	16,170	14,714
Total Estimated Fair Value	$ 44,604	$ 42,187